An ergonomic product based on natural human motion improves user satisfaction and increases efficiency. This study is proposed to develop ergonomic half guard design (groove and projection) which is applied to natural motion for the use of product and validate effect of the development. So, there are three objectives in this study. First is to grasp the characteristics of half guard design and the natural motion for the use of a half guard quantitatively. Second is to develop new half guard design through applying the extracted natural motion. Third is to propose the product design evaluation process used motion analysis and analyze the effect of developed half guard design based on the process. The approach to design a half guard considering user natural motion has 3-stepsdesign characteristics analysis, natural motion analysis, and design improvement. Design characteristics analysis is the step to understand fundamental design characteristics of a half guard (e.g. product components, use environment, and related patents). Natural motion analysis is the step to analyze quantitatively natural human motion for the use of half guard. At first, we capture the natural motion from 18 participants (mean age = 44, SD = 9.0) by motion analysis system, and regression analysis is performed to grasp thetendency of the user natural motion and to extract representative trajectory for applying the user natural motion to half guard design. Design improvement is the step to design a half guard with applying the user natural motion. The improvement process consists of 4-stepsdesign parameter analysis, constraint analysis, design ideation, design determination. Through the 4-steps, 2 types of half guard design are developedTrapezoid wedge with reference point (TrapRP) and Curved wedgeRP (CurvRP). To validate effect of developed half guards in the 3rd step, usability for new (TrapRP, CurvRP) and previous half guard designs (Nipper, NipperRP, Rectangular wedgeRPRectRP) are compared for 18 participants (mean age = 43, SD = 7.7). The usability evaluation is composed to a quantitative measure (trajectory similarityminimum distance between natural motion and trajectory for the use of each half guard) and a qualitative measure (subjective rating for satisfaction of the use of the half guards7 point-scales: totally satisfied, 4: neutral, 1: totally unsatisfied). The developed design includes natural motion and the idea of the reference point (RP). The trajectory of developed designs (CurvRP, TrapRP: 1.0 ± 0.1 cm) are more similar to natural motion than previous designs (NipperRP: 1.6 ± 0.1, RectRP: 1.9 ± 0.1, and Nipper: 2.0 ± 0.1). And participants were highly satisfied with developed designs (CurvRP: 6.3 ± 0.2 point, TrapRP: 5.5 ± 0.3) than previous designs (RectRP: 5.3 ± 0.3, NipperRP: 4.2 ± 0.3, and Nipper: 3.3 ± 0.3).